It is frequently desirable to be able to repeatedly position an instrument, such as an ophthalmic instrument, in a predetermined spatial location relative to an object. For example, a non-contact tonometer must be carefully positioned relative to an eye under test in order to obtain an accurate intraocular pressure reading. It is desirable that the operator be able to position the instrument rapidly, since non-contact tonometers are frequently used to screen a large number of individuals for early warning signs of glaucoma by measuring their intraocular pressure. Instruments, such as non-contact tonometers, must be positioned not only relative to the eye laterally, but also spaced a proper distance axially from the eye. In such instruments, the patient normally places his forehead against a rest and his chin in a cup-like support. The operator then moves the instrument towards the eye to be tested, while observing various indicia until the predetermined location relative to the eye under test is achieved as indicated by the indicia.
The alignment system of the first commercial non-contact tonometer is disclosed in U.S. Pat. No. 3,756,073, issued Sept. 1973 to Lavallee et al. The optical system included a projected target, which the operator centered inside an aiming reticle by looking through an eyepiece, in order to obtain correct lateral positioning. The proper distance from the eye under examination was achieved by moving the instrument toward the patient's eye until the image of the reflected target was observed to be sharply focused. Since operator's were aware that they would not be able to observe the eye through the optical system during the alignment process, they usually performed an initial positioning of the instrument by observing the location of the instrument relative to the eye from the side, while moving the instrument into an approximated proper position. After approximated positioning, the operator then looked through the eyepiece to obtain accurate positioning of the instrument. This procedure avoided inadvertent contact with the eye. A light detector was used to verify the operator's correct alignment before testing.
The miniaturization of electronic components and particularly those relating to television, i.e., cameras and monitors, has permitted adaptation of earlier optical systems to permit the operator to observe the positioning indicia on a CRT screen. U.S. Pat. No. 4,665,923, issued May 19, 1987 is an example of such an alignment system and includes three optical subsystems. Two of the optical subsystems are symmetrically disposed about the instrument axis and provide visible indicia indicating the position of the instrument relative to a predetermined location. The third optical subsystem is used to provide the operator with a macro image of the eye. All of the embodiments disclosed in the patent, as well as the commercial product utilizing disclosed concepts, present the three images to a single observation means, i.e. image pickup tube 53. It is readily apparent that the patented system has the distinct disadvantage that proper adjustment can only be achieved by meticulous adjustment of each component of the two symmetrically disposed systems and manufacturing all components to close tolerances. For example, the first embodiment requires precise alignment of eight reflective surfaces in the two alignment optical subsystems, and even the simplest system, that shown in FIGS. 9 and 10, requires precision alignment of five reflective surfaces. Obviously, the dimensions of each component as well as the mounting thereof and spacing therebetween are extremely critical. An additional disadvantage of the disclosed optical systems is the requirement that at least four of the reflective elements be beam dividers. This substantially reduces the amount of original illumination that can be presented to the image pickup tube. A further disadvantage of the disclosed systems is that optically presenting three separate images to a single camera tube or CCD array causes the macro image of the eye to be washed out or at least very faint. Similar to the earlier system, a spot detector was used to verify correct alignment by the operator before testing in all of the disclosed embodiments.
The criticality in alignment is partly due to the use of a "spot" detector to verify alignment. It is only after light has passed through or been reflected by numerous elements that the spot or spots are evaluated to determine how much light is falling on the detector. introducing errors because of alignment or quality. Another factor affecting the system adversely is the quality of the optics required. Since the disclosed system projects a target image that is imaged on the observation means and the detector, the quality of the final images is controlled by the quality and alignment of all the intervening optical components.
U.S. Pat. No. 4,705,045, issued Nov. 10, 1987, discloses a tonometer alignment system having two oblique target projection systems that re-image the targets through an imaging optical system that is parallel to the discharge tube axis. Only imaging light that is reflected from the eye parallel to the imaging optical system axis is imaged on the detector, with both images being superimposed when the tonometer is aligned properly .